There has been known an inkjet printer that includes a detachably attachable main tank, a sub tank for storing ink supplied from the main tank attached to the inkjet printer, and an image recording unit for recording an image by ejecting ink stored in the sub tank (e.g., Japanese Laid-Open Patent Publication No. 2008-213162). An internal space of the main tank and an internal space of the sub tank each communicate with outside air. Therefore, in response to attachment of the main tank to the inkjet printer, ink moves such that a surface of ink stored in the main tank and a surface of ink stored in the sub tank become at the same level due to a hydraulic pressure difference between the internal space of the main tank and the internal space of the sub tank.